"There are some protocols that Khartoum has not implemented. The people of Abyei have voted unanimously to join South Sudan," he said.

Media reports have said that Sudan's National Election Commission (NEC) plans to include Abyei in the country's 2015 general elections.

Justice Deng Biong Mijak, who is mandated by the government of South Sudan to lead negotiations on Abyei with Khartoum, called the move by the NEC "a clear violation of what is happening between the two countries" on the disputed region.

"Nobody can just unilaterally decide to make a major decision" on Abyei, he said.

The Abyei region was supposed to hold a referendum in January 2011, to decide whether it would be part of South Sudan or Sudan, but residents of Abyei were denied the chance to vote on their future because Juba and Khartoum could not agree on who was eligible to cast a ballot.

The overwhelming majority of residents of Abyei are Ngok Dinka, who are loyal to Juba. Arab Misseriya tribes, who are loyal to Khartoum, migrate through the region and graze their cattle there.

Cipeng Mading Akuei, a paramount chief in Abyei, said his entire family was forced out of the region and lives in Warrap state. Akuei said he would not participate in the Sudanese elections if Khartoum goes ahead and includes Abyei in the vote.

“We have already made our fate in the October referendum," he said, referring to a vote organized unilaterally by the Ngok Dinka, in which an overwhelming majority of those who took part said they wanted to be part of South Sudan. The international community has not recognized the vote, which the Misseriya boycotted.

Juba resident Maguith Deng Kuol, who is originally from Abyei, said he thinks most people in the disputed region would boycott the Sudanese vote.

"Nobody is ready, even if we are invited. Nobody is ready to vote in these elections because we are not part of Sudan. We are not and we will not be part of Sudan at all," he said.

Presidential spokesman Ateny said the situation could be resolved by finally holding a referendum in Abyei to let people choose which country they belong to. He urged Khartoum to let the referendum go ahead.